Electrically conductive asphaltic concrete

ABSTRACT

The passage of an electric current through an electrically conductive asphaltic concrete surface generates sufficient heat within the surface to prevent the accumulation of snow and ice thereon. The asphaltic concrete is made electrically conductive by incorporating graphite particles within the concrete mix.

United States Patent lnventor Appl. No.

Filed Patented Assignee Louis David Minsk Hanover, N.H.

July 30, 1969 Apr. 6, 1971 The United States of America as representedby the Secretary 01 the Army ELECTRICALLY CONDUCTIVE ASPHALTIC CONCRETE5 Claims, No Drawings U.S. Cl 219/213, 252/503 Int. Cl H05b 3/60 FieldofSearch 219/213, 19.4,

[56] References Cited UNITED STATES PATENTS 2,314,766 3/1943 Bull et a1219/213 3,047,701 7/1962 Frungel 219/213 3,166,518 1/1965 Barnard252/511X Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. .laegerAttorneys-Harry M. Saragovintz, Edward J. Kelly, Herbert Berl andLawrence E. Labadini ELECTRICALLY CONDUCTIVE ASPHALTIC CONCRETE Theinvention described herein if patented, may be manufactured and used byor for the Government for governmental purposes, without the payment tome of any royalty thereon.

This invention relates to the generation of heat electrically in anasphaltic concrete pavement or other surface. More particularly, thisinvention relates to the prevention of the accumulation of ice and snowon pavement by use of an asphaltic concrete made electrically conductiveby the addition of graphite particles thereto and by the passage of anelectrical current through such asphaltic concrete to generatesufficient heat to melt the ice or snow.

BACKGROUND OF THE INVENTION Present practical methods of control of snowand ice accumulation on paved surfaces can be classified as chemical,mechanical and thermal. Melting of frozen precipitation by heat can beaccomplished by direct application of thermal energy from an exposedflame or an electrically energized radiant source, by pipes carrying hotliquid or by electrical resistance cables buried in the upper portion ofthe pavement. The application of heat from above the surface by radiantenergy requires the melting of the entire ice or snow mass to effectremoval, a method that consumes large quantities of energy. The buriedelectrical cable method is preferable since it enables the heat to beapplied more efficiently to the snow or ice than the other methods.However, there are drawbacks to the use of buried heating cables. Eitherthe spacing between the cables must be very small or the temperature ofthe cables must be very high to obtain adequate heat input to melt snowor ice in the areas between them. Furthermore, cables must be buriedrelatively deep in the pavement to obtain the optimum distribution ofheat for a given electrical input and cable size. This requires a majorconstruction job for placement of the cables as well as the undesirabletask of breaking the pavement surface in old construction. The use ofimbedded pipes carrying hot fluid is subject to the same disadvantagesas set forth for buried electrical cables. Additionally, if repair workon the pipes or cables is required, then the pavement must be torn upwhich is both costly and disruptive of normal operations on the pavedsurface.

SUMMARY OF THE INVENTION I have discovered a novel method and novelmaterials which make it possible to generate heat uniformly andefficiently at the ice-pavement interface to effect separation of icewith a minimum energy or to effect melting of snow or ice. The heat isgenerated by passage of an electrical current through an asphalticconcrete pavement layer having a resistivity within the range from about1 to about 5 ohm-inch. Asphaltic concrete paving material having thedesired resistivity characteristics is prepared by incorporating withinconventional asphaltic concrete mixes a quantity of graphite particles.Electricity is carried to the conductive asphaltic concrete pavementlayer by conductor busses or cables spaced widely apart (3 to feet ormore, depending on the voltage gradient selected). Such electricallyconductive paving material can be easily applied as a thin continuousoverlay to existing pavements avoiding the type of major constructioninvolved in burying cables or pipes in existing pavements. Repair workis easily accomplished by patching with a mix of the same electricallyconductive material. In addition, since there would be significantlyfewer electric cables or pipes used there would be a concomitantreduction in the likelihood of damage to such cables.

Accordingly, it is among the objects of the present invention to providea method and means for efficiently generating heat at the surface of anasphaltic concrete pavement to prevent accumulation of snow or icethereon. Other objects will become apparent in the following detaileddescription of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS concrete, also known asbituminous concrete is a widely available article of commerce whichvaries somewhat in the percentage of its components. Such concreteconsists, in the main, of a sand, crushed stone or gravel aggregatecombined with an asphalt cement binder. The properties of the resultingconcrete surface will depend on the relative proportions of sand andcrushed stone or gravel as well as the size of the stone and gravel. Theasphalt cement acts as a binder for the aggregate and generallycomprises from 5 to 15 percent or more by weight of the composition.High purity graphite is graphite containing percent or more pure carbonand 10 percent or less of ash or volatiles. Graphite particles suitablefor use in the present invention range in size from particles no largerthan those which can completely pass through a No. 4 Sieve and nosmaller than particles which can pass through a No. 200 Sieve, saidSieve numbers being in the US. Standard Sieve Series. The following twoExamples illustrate the preparation of typical asphaltic concretecompositions according to the present invention.

EXAMPLE I An asphaltic concrete mixture containing 276 lbs. 9ll6-inchcrushed stone, 550 lbs. %-inch crushed stone, 642 lbs. of sand and I66lbs. of asphaltic cement (l00l20 penetration) was prepared according totechniques known in the art. 366 lbs. of a high purity graphite (98percent carbon, 2 percent ash and volatiles) particles, which will pass100 percent of the particles through a No. 4 Sieve and 0 percent througha No. 20 Sieve was added to a pug mill after the aggregares had beenbatched and thoroughly blended in the hot asphaltic concrete mixturewhile the mix is maintained at a temperature of 350 F.

EXAMPLE II In this example, 450 lbs. of high purity graphite particles(95 percent pure carbon), particle size being such as to completely passthrough a No. 65 Sieve and only 41.2 percent to pass through a No. 200Sieve, preheated to a temperature above F. were added to an asphalticconcrete hot mix in a pug mill. The concrete mix consisted of lbs. ofzinch crushed gravel, 630 lbs. of a %-inch crushed gravel, 540 lbs. sandand 200 lbs of asphalt cement (85100 penetration). The graphite wasthoroughly blended in the hot mix and the temperature of the completedmix as discharged from the mill was within the range of 275 F. to 325 F.

Pavement or other asphaltic concrete surfaces are constructed in amanner well known in the art which consists generally of spreading thehot mix uniformly over a suitable base followed by compacting. Thethickness of surfaces formed with the asphaltic concrete compositions ofthis invention may be varied within wide limits. Because of costconsiderations, however, we prefer not to exceed 2 inches in thicknessand for durability, we prefer not to have a surface less than one-halfinch in thickness. If the surface is expected to be subjected to heavywear, it is desirable to cover the electrically conductive surface witha nonconductive wear course of from one-half inch to l- /zinches inthickness. Such a wear course would also serve as a protective surfacecoating to prevent large increases in current flow caused by metalconductors falling across or penetrating the conductive asphalticmaterial. It is, of course, desirable to have the conductive asphalticconcrete surface as uniform in thickness as possible so as to avoid hotor cold spots in the pavement.

As the conductive concrete composition is spread over the surface to becovered, copper conductor cables are placed within this layer ofmaterial. The cables are spaced at regular intervals and connected to asuitable voltage source so that the desired electrical potential may bemaintained between the copper conductors.

tion, copper conductors were placed in the conductive concrete materialand spaced feet apart. Conductors were connected to center taptransformers which, in turn, were connected to auto transformers forvoltage control. 60 cycle In operation, the power dissipation requiredto prevent the 5 AC current was used to supply the transformers. InTable l, accumulation of ice and snow should fall within the range ofthe thickness of the electrically conductive concrete material to 40watts per square foot. and the gauge of the copper conductors used areidentified for Power is consumed when current flows through a purelyreeach hole or panel. Table [1 demonstrates the effectiveness of sistiveload under an applied potential according to the relapanels preparedaccording to my invention in clearing snow tion (Equation 1) 10 and icefrom the paved surface over a period of time during E2 which thesurfaces were covered with fresh snow of varying P=EI= depths. The poorresults for panel No. 2 are traceable to the fact that the powerdissipation of this panel range between 3 where and 7 watts per squarefoot, below the desired range of 10 to P= power dissipated (w) 40 wattsper square foot.

E applied potential difference (v) I= current (amp) TABLE I R resistance(ohm) Gage of Material exhibit a resistance directly proportional to theThickness, copper length of the conducting path and inverselyproportional to PanelN0- Conduct the cross-sectional area of theconducting element, A or 1 A #10 2 #6 (Equation 2) 3 1 #6 1 z i 1? ii p2 6 1 2 R A, tw 2s yz TABLE II New Wind Air Percent clear of panel snow,speed, temp., Date in. m.p.h. F. 2 3 4 5 6 12/20/65 Ca1m 7 100 0 100 8090 100 1/13/67 2 undo. 27 100 95 100 100 100 100 2/2/67 #4 do. 34 100slush 100 100 100 100 2/21/67 5 .do 23-28 100 50 100 100 100 100 3/6/67--do 29 100 0 100 100 100 100 3/16/67 4 5 26 100 Oice 100 100 100 100Where Iclaim:

R resistance (ohm) p proportionality constant, resistivity (ohm-in.) Iconducting path length (ft.) t= thickness of conducting sheet (in.)

w width of conducting sheet (ft.)

Substituting eq. 2 to eq. 1 gives Power dissipation per unit surfacearea, A is E tw E 2 t P/Ar wl I (2 For safety reasons, it is preferredthat the potential drop between electrodes not exceed 30 volts. While wehave found an approximate 5 foot spacing between electrodes to bepreferable, since this establishes a potential gradient of 6 volts perfoot, other spacings (3- l 5 feet) are possible provided the 30 voltpotential between electrodes not be exceeded.

EXAMPLE llI Six 6"X6 holes or panels were cut in an existing asphaltparking lot and backfilled with sand and a standard asphaltic concretehot mix to give two holes each having an unfilled depth of one-halfinch, 1 inch and l /zinches. The conductive asphaltic concretecomposition of Example I was poured into each of the holes up to gradeand compacted. Prior to comple- 2. A method according to claim 1 whereinthe current is passed through the electrical conductive asphalticconcrete by spaced-apart electrodes imbedded within the concrete.

3. A method according to claim 2 wherein the potential differencebetween the electrodes does not exceed 30 volts.

4. A method according to claim 3 wherein the power dissipated withinsaid electrical conductive layer of asphaltic concrete ranges from 10 to40 watts per square foot.

5. A method according to claim 4 wherein said electrically conductiveasphaltic concrete consists of asphaltic concrete mixes having dispersedtherein high purity graphite particles, said graphite particlesconstituting from 20 percent to 30 percent by weight of the mixturebased on the total weight of the concrete aggregates.

2. A method according to claim 1 wherein the current is passed throughthe electrical conductive asphaltic concrete by spaced-apart electrodesimbedded within the concrete.
 3. A method according to claim 2 whereinthe potential difference between the electrodes does not exceed 30volts.
 4. A method according to claim 3 wherein the power dissipatedwithin said electrical conductive layer of asphaltic concrete rangesfrom 10 to 40 watts per square foot.
 5. A method according to claim 4wherein said electrically conductive asphaltic concrete consists ofasphaltic concrete mixes having dispersed therein high purity graphiteparticles, said graphite particles constituting from 20 percent to 30percent by weight of the mixture based on the total weight of theconcrete aggregates.